A secondary battery is a battery that can be repetitively recharged. The charging characteristics of a secondary battery including the behavior of the battery terminal voltage as charging progresses often depends upon the type of the secondary battery. For example, a nickel-cadmium battery has charging characteristics in which the terminal voltage of the secondary battery typically peaks when the battery is fully charged. On the other hand, nonaqueous secondary batteries, such as, for example, a nonaqueous, organic electrolyte, lithium secondary battery (hereinafter referred to as a lithium ion secondary battery), usually does not exhibit a peak of the terminal voltage. Instead, the terminal voltage usually continues to increase as the battery is overcharged. The latter charging characteristics of nonaqueous secondary batteries can make detection of the fully charged state more difficult.
In addition, when secondary batteries of any type are overcharged, the properties of the secondary batteries are usually deteriorated. For example, the life span of the secondary battery is typically decreased by an overcharge. A lithium ion secondary battery is particularly susceptible to weakening as a result of excessive charging.
To prevent the lithium ion secondary battery from being overcharged to an excessively high voltage, U.S. Pat. No. 5,028,860 describes a method of charging using a constant voltage. When the lithium ion secondary battery is charged with a constant voltage, the charging current typically decreases, as the charging progresses to the full charge level. Accordingly, the charging current may be monitored, and when the charging current becomes smaller than a predetermined current value, the charging current can be shut off. However, it is often difficult to determine the appropriate current value at which to terminate the charging current. For example, in order to fully charge a lithium ion secondary battery, it is usually necessary to set the predetermined current value to a small value. However, when the lithium ion secondary battery becomes degraded or is charged at a low ambient temperature, the charging current often does not become sufficiently small even after the battery is fully charged. Therefore, if the predetermined current value is set to too small of a value, the charging current may not ever reach the predetermined current value under a variety of circumstances such that charging may not be properly terminated.
Japanese Patent Laid-open No. HEI 3-253232 describes a charging method in which the terminal voltage of the secondary battery being charged is monitored to detect the fully charged state. As shown in FIG. 1, soon after the charging process is started, the charging current is reduced during a predetermined interval to a small charging current value (approximately zero charging current) which is substantially smaller than the normal charging current value. During this interval, the terminal voltage is approximately the same as the open circuit voltage which provides a good indication of the charge level of the battery. On the other hand, when the terminal voltage is monitored during the normal charging current, the monitored terminal voltage is usually much larger than the open circuit voltage because of a voltage drop caused by contact resistance or by an internal resistance within the secondary battery.
The method described in HEI 3-253232 reference facilitates the correct detection of the fully charged state. However, this method can increase the time necessary to fully charge the secondary battery, because the secondary battery is charged with a small charging current during the predetermined interval following the initiation of charging.